1. Field of the Invention
The present invention relates to a digital image signal encoding device, a digital image signal decoding device, a digital image signal encoding method, and a digital image signal decoding method used for an image compression encoding technology or a compressed image data transmission technology.
2. Description of the Related Art
An international standard video encoding system such as MPEG or ITU-T H. 26x has conventionally been premised on use of a standardized input signal format called a 4:2:0 format. The 4:2:0 format is a format where a color moving image signal of RGB or the like is converted into a luminance component (Y) and two chrominance components (Cb, Cr), and the number of chrominance component samples is reduced to half of luminance components both in horizontal and vertical directions. The chrominance component is inferior to the luminance component in visibility. Accordingly, the conventional international standard video encoding system such as the MPEG-4 AVC (ISO/IEC 14496-10)/ITU-T H.264 standard (hereinafter, referred to simply as AVC) (hereinafter, referred to as Non-patent Document 1) has been based on the premise that the amount of original information to be encoded is reduced by downsampling chrominance components before encoding is executed as mentioned above. On the other hand, with recent increases in resolution and gradation of a video display and for the purpose of precisely reproducing on the screen color representation at the time of creating contents of digital cinema and the like, studies have been made on a system for performing encoding by maintaining the number of samples equal to that of luminance components without downsampling chrominance components. A format where the numbers of luminance and chrominance component samples are completely equal is called a 4:4:4 format. According to Non-patent Document 1, a “high 4:4:4: profile” has been developed as an encoding method which uses the 4:4:4 format as an input. As a method suited for this object, there is employed a standard method as defined in the JPEG 2000 (ISO/IEC 15444) standard (hereinafter, referred to as Non-patent Document 2). As illustrated in FIG. 10, the conventional 4:2:0 format has been limited to Y, Cb, and Cr color space definitions because of the premise of downsampling of chrominance components. In the case of the 4:4:4 format, however, because there is no sample ratio distinction between color components, R, G, and B can be directly used in addition to Y, Cb, and Cr, and a plurality of color space definitions can be used. In a video encoding method using the 4:2:0 format, a color space is limited to a Y, Cb, and Cr color space. Therefore, the type of color space is not required to be taken into consideration during an encoding process. In the AVC high 4:4:4 profile described above, however, color space definition affects the encoding process itself. On the other hand, because the current high 4:4:4 profile considers the compatibility with the other profiles which use the 4:2:0 format defined by the Y, Cb, and Cr color space as a target to be encoded, it is not considered that the high 4:4:4 profile is designed to optimize a compression efficiency of the 4:4:4 format.
For example, in a high 4:2:0 profile encoding the 4:2:0 format of the AVC, in a macroblock area composed of luminance components of 16×16 pixels, corresponding chrominance components are 8×8 pixel blocks for both Cb and Cr. In motion compensation prediction of the high 4:2:0 profile, block size information which becomes a unit of motion compensation prediction only for the luminance components, reference image information used for prediction, and motion vector information of each block are multiplexed, and motion compensation prediction is carried out for chrominance components by the same information as that of the luminance components. The above method is premised on the color space definition that the contribution of the chrominance components is less than that of the luminance component which contributes greatly to the expression of an image structure (texture) in the 4:2:0 format. However, the current high 4:4:4 profile corresponds to the simple extension of the intra prediction mode for chrominance in the 4:2:0 format even when the block size of a chrominance signal per macroblock is expanded to 16×16 pixels. Moreover, as in the case of the 4:2:0 format, one component is regarded as the luminance component. After only information for one component is multiplexed, motion compensation prediction is performed using an inter prediction mode, reference image information, and motion vector information which are common to three components. Therefore, the prediction method is not always optimal for the 4:4:4 format in which the respective color components equally contribute to the expression of the structure of an image signal.